1. Field of the Invention
The present invention relates to an imager apparatus.
2. Description of the Related Art
Hitherto, a technique of shielding dust from optical apparatuses has been proposed. The technique consists in vibrating a protective glass plate (glass dust shield) to remove dust from the glass dust shield. An example of this technique is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-204379. In the example, a piezoelectric element is used as means for vibrating the glass dust shield. The piezoelectric element expands and contracts in response to the voltage applied on it. It vibrates the glass dust shield attached to it, at a predetermined frequency.
Electronic imager apparatuses can operate in various modes. The user uses an electronic imager apparatus in various conditions. Among the operation modes of the electronic imager apparatus is rapid-sequence photographing mode. In the rapid-sequence photographing mode, the apparatus can acquire image data items, one after another, as long as the release SW remains closed. In this case, it is desired that the apparatus should perform a photographing process after dust is completely removed. It is therefore necessary to vibrate the glass dust shield for each photographing process. As the shield is so vibrated, the release-time lag increases, resulting in a decrease in the speed of rapid-sequence photographing. Consequently, the apparatus is not user-friendly.
It is demanded that cameras make but as little noise as possible while they perform the photographing process. To meet the demand, a camera system has been proposed, which can operate in a specific mode (hereinafter called “low-noise operating mode”) to perform photographing process quietly.
In the low-noise operating mode, the camera system makes no audio messages to the user and mechanical components of the camera are so driven as to generate no noise.
When the glass dust shield is vibrated to remove dust from it, air vibrates, making noise. If the air vibrates at a frequency that falls in the audio range, it makes a noise. The user hears the noise. The user does not want to hear any noise while the camera system remains in the low-noise operating mode.
Dust may stick to the glass dust shield in various conditions. It may enter the camera and may stick to the glass dust shield when the mirror box is exposed in the course of exchanging lenses. It may more probably stick to the glass dust shield when the shutter is opened without attaching the exchange lenses (or lens unit) to the camera. Even is such a case, it may not so problematical if the shutter speed is high.
Now that cameras have a high-performance imaging element, some users attach them to telescopes to take astrographs. To take an astrograph, the shutter must be held open for a long time because the object luminance is low. Dust may stick while the shutter remains open. Most telescopes are much larger than exchange lenses and less shielded. Hence, the glass dust shield is more likely to contact dust-containing air than in the case of telescopes.
Electronic imager apparatuses can operate in a mode to photograph an object in various image qualities (number of pixels recorded, ratio of image compression, and the like). The user can photograph an object in any image quality desirable for the use of the resultant image data, for example, to be processed by personal computers or be used in preparing homepages. The user can acquire high-quality image data, by removing dust from the glass dust shield.
If the user selects low-quality image data that dust scarcely influences, however, it is useless to remove dust from the glass dust shield. In a particular case, the user may want to take as many pictures as possible at the expense of image quality. Then, it is not desirable to remove dust each time a picture is taken. As dust is repeatedly removed, the battery that supplies power to the camera system is deprived of more electric energy. This may disable the user to take a desired number of pictures.
In another specific case, the user may try to shorten the time for rapid-sequence photographing, at the expense of image quality. He may reduce the number of pixels recorded for each image, thereby to shorten the time required to access the medium, which stores the image data. If performed in this condition, the removal of dust will increase the release-time lag. Consequently, the rapid-sequence photographing speed decreases, against the user's intension.